How to lower alpha in dark areas ?

I’m making images of a fire and need to have the lit area show, but turn alpha transparent further from the fire. How can I make the darker ground alpha 0, but get the lightened area near to stay?
I can make the ground image first and calc alpha when image is applied to the floor, but I thought there was a more direct way.

To what are you applying alpha?

If you’re talking about the light cast from the fire, each light has distance and drop-off controls (select your light object and press F5) that allow you to control how far the light shines and how fast it decays.

Does this help?

Thanks, but no.
I’ll need to paste the image over a landscape in another application, and I need the landscape to show and have the fire add extra light without a big black square.
I’ll just use a map with caclulated alpha.

Oh, I see.

For best results, you can model an approximation of the target landscape, give it a Lambert pure white (with no specularity) and render, then use that for your alpha.

Good luck.